This invention relates to a fixing device and an image forming apparatus, and particularly relates to a temperature controlling system of a heating member heated by a heat source.
In order to control the surface temperature of a heat roller (i.e., a heating member), a conventional fixing device has a non-contacting temperature sensor in the proximity of the outer surface of the heat roller, and turns on and off a heat source of the heat roller according to the temperature detected by the non-contacting temperature sensor. The detected temperature of the heat roller is compensated based on a printing condition (for example, a continuous printing operation), a change in the detected temperature (increasing or decreasing) or the like. Such an image forming apparatus is disclosed by, for example, Japanese Laid-Open Patent Publication No. 2001-242741 (see page 1 and FIG. 1).
However, because of the influence of the ambient temperature (for example, the temperature of a cover of the fixing device), the difference between the detected temperature detected by the non-contacting temperature sensor and the actual surface temperature of the heat roller may deviate, and therefore incorrect detection of the temperature may occur. In such a case, it is difficult to compensate the incorrect detection of the temperature.